1. Field of the Invention
The present invention generally relates to a microstripline. More particularly, the present invention relates to a microstripline for use in a microwave heating apparatus, for example, adapted for uniformly heating a sheet-like material such as a paper sheet and a surface of other type of a material being heated.
2. Description of the Prior Art
Conventionally, a microwave heating apparatus using a microstripline in which a ladder pattern is formed in its ground conductor has been known for microwave heating of a thin material being heated such as a paper sheet. For example, the Japanese Patent Laying Open Gazette No. 14488/1983 laid open Jan. 27, 1984, discloses such microwave heating apparatus. FIG. 1 is a perspective view showing one example of such microwave heating apparatus. The microwave heating apparatus comprises a microstripline 1, a coaxial line 7 for supplying a microwave to the microstripline 1, and a dummy load 8. The microstripline 1 comprises a dielectric base plate 2 made of a ceramic of alumina, for example, and a wider ground conductor 3 is formed on the surface of the dielectric base plate 2. The ground conductor 3 is formed of an electrically good conductive material, such as copper and silver, and a ladder pattern 6 is formed in the part of the ground conductor 3 in the length direction. The ladder pattern 6 comprises a plurality of leakage openings or slits 5 arranged distributed in the length direction, i.e. the propagating direction of a microwave. The microstripline 1 further comprises a center plane or center conductor 4 made of silver or copper, for example, formed on the rear surface of the dielectric base plate 2 so as to be adhered thereto. Normally, the ground conductor 3 and the center conductor 4 are formed of copper plate or silver plate, the thickness of which is 0.2 mm, respectively. The width of the slits 5 in the ladder pattern 6 formed in the ground conductor 3 is 4 mm and the length thereof is about 30 mm. A microwave oscillator of such as a magnetron, not shown, is provided at the input side, i.e. at the left side as viewed in FIG. 1, of the coaxial line 7, so that the coaxial line 7 is supplied with microwave from the microwave oscillator to supply the same to the microstripline 1. A dummy load 8 is connected to the side opposite to the input side of the microwave of the microstripline 1. The dummy load 8 is aimed to absorb and consume a microwave not consumed by the ladder pattern 6, thereby to protect the microwave oscillator.
A microwave is supplied to the microstripline 1 through the coaxial line 7 upon energization of the microwave oscillator, not shown, in the above described structure. A portion of the supplied microwave is leaked through the respective slits 5 at the ladder pattern 6 formed in the ground conductor 3. Accordingly, a sheet-like material being heated 9 such as a paper sheet placed on the ladder pattern 6 is heated by the leaked microwave. Meanwhile, by providing a transfer means such as a conveyor or rollers as shown, such that the material being heated 9 is in succession transferred in the arrow direction, the material being heated 9 is in succession and continually heated.
On the other hand, the microstripline 1 for use in a conventional microwave heating apparatus has been produced in the following method. More particularly, as shown in FIG. 2, first, a plurality of slits 5 are punched, by a press, in a copper plate which is the ground conductor 3, thereby to form the ladder pattern 6. Thereafter, the ground conductor 3 having the ladder pattern 6 and the center conductor 4 are pressed while heating, so as to sandwich the dielectric base plate 2.
Normally, a microwave of about 800 W is applied to the above described microstripline 1 for use in the microwave heating apparatus. However, if and when such microwave of high power is applied to the microstripline 1, it is most likely that a spark occurs between the lattice portions 3a. For example, in case where the dielectric base plate 2 is formed of polyethylene tetrafluoride sheet of 0.1 mm in thickness, which has a low corona resistance, and an equivalent life time to dielectric breakdown is about 35 hours, with 50.degree. C. of ambient temperature and an applied voltage being 3 kV with 50 Hz of frequency, and mechanical breakdown occurs ten seconds after such plate is in an arc. Thus, in general, a spark readily occurs in a microstripline 1 as shown in FIGS. 1 and 2 and hence the dielectric base plate 2 is likely to be broken due to such spark.
Then, in order to solve such problems, for example, the Japanese Patent Laying Open Gazette No. 101374/1982 (Japanese Patent Application No. 178333/1980) laid open June 23, 1982 discloses that one surface of a ladder pattern is covered with an insulating material, such as silicone resin and a glass. The Japanese Patent Laying Open Gazette No. 154791/1982 (Japanese Patent Application No. 41087/1981) laid open Sept. 24, 1982 discloses that one surface of a ladder pattern is provided with a cover made of the same insulating material, such as silicone resin, fluororesin and glass. However, in these examples, a conductor having a ladder pattern is directly disposed on a dielectric base plate and thereafter, the exposed surface of the conductor having a ladder pattern is covered with an insulating material or is provided with the cover made of an insulating material. Accordingly, a creeping discharge is caused between the interfaces of the conductor having the ladder pattern and the dielectric substrate, which means that there is still a fear that leakage may occur. Therefore, the above described solution or solutions are not sufficient for a perfect spark prevention. In addition, a coating of an insulating material over only one surface of a conductor having ladder pattern tends to be stripped off. In case of using a cover made of an insulating material, such approach has difficulty of providing precisely such cover on the ladder pattern, which makes the yield worse.